motorboating tube

Last year I bought a quad of Genalex Gold Lion KT-88's and a Gold Lion tube rectifier for my VTA ST-120 amp I purchased from Bob Latino built by a Latino technician. I installed them, biased them and was sitting back enjoying the music and in a short time one went bad. Jim McShane replaced it yet some how made it my fault that he wasn't able to get it replaced under warranty because I supposedly didn't shut the amp down in time and he wasn't able to test the tube. I was also some how guilty of not checking to make sure the socket contacts were clean and making good contact with the tubes.

I put the TungSol KT-120's and the Weber solid state rectifier back in the amp and hadn't had a problem until one of the TungSol's started glowing a lot brighter than the others. The tube that glowed brighter was in the front left socket. I figured since the tubes probably had 2000 hours on them it was time for a replacement. So I put the Sovtek KT-88's that I got back in the amp earlier this week. Everything worked without a problem.

Today I decided that since these two other tube types worked fine the Gold Lions I got last year should be good too since McShane replaced the bad tube. As it turns out before I could even bias the Gold Lions I heard a motorboating sound. I quickly shut the amp down. I also noticed that the Gold Lion KT-88 in the right front socket was warmer than the others.

So...what's the deal here?

Always the same socket on the amp giving you issues? Tube socket contact, bias pot wiper losing contact, and leaky coupling caps can be cause of the issues described. The 1K ohm grid stopper resistors on the power tube socket pin 5 are supposed damp self oscillation. Jim McShane stated the grid stopper value may need to be higher when KT-120s are used. But that is something else to check.

No...the KT-120 was in the left front. The Gold Lion that got warmer than the rest was in the right front.

So after posting this first message I took the Gold Lions out and cleaned the contacts with alcohol and installed the Sovteks and Weber rectifier in again. The amp started up fine and I let it play for around a half hour. Shut it down and let it cool off. I put the Gold Lion KT-88's in, biased them and the amp played fine. I shut it down and let the tubes cool off and put the Gold Lion rectifier in. Started the amp and let it warm up for a few minutes and now everything seems ok. The amp has been on and cranking out tunes for 15 minutes or so.

Could the socket contacts have been dirty or corroded and with removing and replacing tubes along with cleaning them removed the dirt and corrosion?

Dogstar wrote:No...the KT-120 was in the left front. The Gold Lion that got warmer than the rest was in the right front.

So after posting this first message I took the Gold Lions out and cleaned the contacts with alcohol and installed the Sovteks and Weber rectifier in again. The amp started up fine and I let it play for around a half hour. Shut it down and let it cool off. I put the Gold Lion KT-88's in, biased them and the amp played fine. I shut it down and let the tubes cool off and put the Gold Lion rectifier in. Started the amp and let it warm up for a few minutes and now everything seems ok. The amp has been on and cranking out tunes for 15 minutes or so.

Could the socket contacts have been dirty or corroded and with removing and replacing tubes along with cleaning them removed the dirt and corrosion?

Excellent!  Yes, these amp circuits are so high impedance that sometimes it doesn't take much to greatly affect performance.  Bad solder joints, components out of tolerance, socket contact, etc.  Plus, pin diameter between tube manufacturers varies as well.  I've put a lot of tubes through my ST-120 and try to clean and re-tension the socket contacts about once a year.

I would though check bias frequently until satisfied everything is stable.

Dogstar wrote:Last year I bought a quad of Genalex Gold Lion KT-88's and a Gold Lion tube rectifier for my VTA KT-88 amp I purchased from Bob Latino built by a Latino technician. I installed them, biased them and was sitting back enjoying the music and in a short time one went bad. Jim McShane replaced it yet some how made it my fault that he wasn't able to get it replaced under warranty because I supposedly didn't shut the amp down in time and he wasn't able to test the tube. I was also some how guilty of not checking to make sure the socket contacts were clean and making good contact with the tubes.

I put the TungSol KT-120's and the Weber solid state rectifier back in the amp and hadn't had a problem until one of the TungSol's started glowing a lot brighter than the others. The tube that glowed brighter was in the front left socket. I figured since the tubes probably had 2000 hours on them it was time for a replacement. So I put the Sovtek KT-88's that I got back in the amp earlier this week. Everything worked without a problem.

Today I decided that since these two other tube types worked fine the Gold Lions I got last year should be good too since McShane replaced the bad tube. As it turns out before I could even bias the Gold Lions I heard a motorboating sound. I quickly shut the amp down. I also noticed that the Gold Lion KT-88 in the right front socket was warmer than the others.

So...what's the deal here?

I don't recall all the details of your situation but you've made an accusation that is dubious at best. I NEVER expect to be able to test a failed power tube and any warranty decision had nothing to do with shutting down the amp in time. I bend over backwards to provide warranty coverage whenever possible. In fact, for a long time I provided an extended warranty on the Genalex KT-88 and funded it out of my own pocket. I do recommend that tube sockets be cleaned (and retensioned if necessary) whenever tubes are replaced. The HUGE majority of tubes I replace under warranty go on to test/play just fine. Why? Because the #1 cause of issues with tube replacement is poor pin to socket contact. If you considered that to be offensive I'm sorry. You and other readers may find this FAQ post in the Tubes Asylum to be helpful:

http://www.audioasylum.com/cgi/d.mpl?audio/faq.html#tubes

So if I did not replace the failed tube free of charge a year ago I will replace it now at no charge to you whatsoever, even if poor contact was the cause of trouble. Just contact me with the shipping address and the cathode current and Gm info from the tube so I can send a tube that is a decent match. Or I will give you store credit equal to the value of the tube. Your call.

Also, I hope you knew to reset the bias when you changed the rectifier tube - that is also a must-do!

arledgsc wrote:Always the same socket on the amp giving you issues?   Tube socket contact, bias pot wiper losing contact, and leaky coupling caps can be cause of the issues described.  The 1K ohm grid stopper resistors on the power tube socket pin 5 are supposed damp self oscillation.  Jim McShane stated the grid stopper value may need to be higher when KT-120s are used.  But that is something else to check.

Actually it is not the grid STOPPER -  it's the grid "leak" resistor. The grid stopper is in series with the tube grid; the grid "leak" resistor goes from the tube grid to ground or to the negative voltage of the bias supply. The spec for the KT120 calls for a maximum "Grid 1 circuit resistance" (essentially the grid leak resistor value) for a fixed bias circuit of 51K Ohms. A KT-88 has a spec of 100K. Exceeding these values puts the tube at risk, it can cause the tube to run away.

It sounds like I didn't do a real good job of communicating that to you, my apologies.

Yes...I removed the Weber rectifier and installed the Gold Lion rectifier. All of the bias pots were turned all the was down before I powered up the amp again and then re-biased them to .500 VDC. So far so good. The Gold Lions have about 4 hours of continuous runtime on them and they sound good.

I have a question regarding tube rolling. If I remove the Gold Lions and install a quad of identical brand and model tubes do the Gold Lions need breaking in again if I reinstall them. I know the answer is probably no but I'm asking because when I bought a pair of speakers that were custom built using used drivers the builder told me I needed to let them break in before their sound improved. Seemed odd fir him to say that but it was true. After about 10 or so hours they did begin to sound better. Or am I just hallucinating?

Dogstar wrote:Yes...I removed the Weber rectifier and installed the Gold Lion rectifier. All of the bias pots were turned all the was down before I powered up the amp again and then re-biased them to .500 VDC. So far so good. The Gold Lions have about 4 hours of continuous runtime on them and they sound good.

I have a question regarding tube rolling. If I remove the Gold Lions and install a quad of identical brand and model tubes do the Gold Lions need breaking in again if I reinstall them. I know the answer is probably no but I'm asking because when I bought a pair of speakers that were custom built using used drivers the builder told me I needed to let them break in before their sound improved. Seemed odd fir him to say that but it was true. After about 10 or so hours they did begin to sound better. Or am I just hallucinating?

You're not hallucinating.

http://www.gr-research.com/burnin.htm

corndog71 wrote:

Dogstar wrote:Yes...I removed the Weber rectifier and installed the Gold Lion rectifier. All of the bias pots were turned all the was down before I powered up the amp again and then re-biased them to .500 VDC. So far so good. The Gold Lions have about 4 hours of continuous runtime on them and they sound good.

I have a question regarding tube rolling. If I remove the Gold Lions and install a quad of identical brand and model tubes do the Gold Lions need breaking in again if I reinstall them. I know the answer is probably no but I'm asking because when I bought a pair of speakers that were custom built using used drivers the builder told me I needed to let them break in before their sound improved. Seemed odd fir him to say that but it was true. After about 10 or so hours they did begin to sound better. Or am I just hallucinating?

You're not hallucinating.

http://www.gr-research.com/burnin.htm

But the tubes don't need any more break in -  although if you've listened to another tube in place of the GLs for a while your ears will likely need a bit of time to adjust.

Speakers are mechanical devices - and as such, breaking in will be a very real part of how they age, settle, and then age some more as with any other mechanical device.

Tubes are electrical devices with a very short break-in period - so short, in fact, as to be as long as it takes to flash the 'getter' and then run the tube for a few minutes to burn off any manufacturing residues. After which, they begin to age as any other devices with a filament will age. Keep in mind that the getter is flashed in the factory, and the typical tube is tested at least for a few seconds (perhaps not if the tube is from China) prior to shipment. Meaning that after about 5 or less minutes of actual use, it is as broken in as it will ever be.

likely the biggest problem here is that not all tubes (even the same type!) have the same size pins!!
sockets need to be tightened if replacing tubes.
Also, what Jim said is another big problem . . . KT120 calls for a grid leak resistor value of 51K Ohms. A KT-88 has a spec of 100K.
Bob's VTA70 amps typically come with 150K (for EL34s), so those four resistors may need to be changed to a lower value for bigger tubes. Better a lower value to be safe anyway.
It just means slightly more bias to make up for more current loss.